Method for producing bearing grade alloy steels

ABSTRACT

This invention is an improvement in a method for producing alloy steel which may contain, for example about 1.00% carbon, about 0.45% manganese, about 0.25% silicon and 1.50% chromium. The steel has a lower incidence of nonmetallic inclusions than steel produced in a conventional method, for example an air-melt electric furnace, open hearth. The steel is melted and tapped and then vacuum treated by any one of several well-known methods. The steel is deoxidized and a fluoride-containing acid slag is added to the steel. The steel and the slag are intimately mixed. The steel is then teemed into molds which can be relatively large.

United States Patent Lehman Feb, 1, 1972 [54] METHOD FOR PRODUCINGBEARING GRADE ALLOY STEELS [21] Appl. No.: 37,297

[52] US. Cl.. ..75/49, 75/58, 75/94 [51] Int. Cl. ..C2lb 7/10, C21b 7/06[58] Field of Search ..75/49,58,94; 164/55, 56, 164/61 [56] ReferencesCited UNITED STATES PATENTS 2,510,155 6/1950 Tanczyn .164/56 2,848,3178/1958 Coupette et a1. .....75/49 2,993,780 7/1961 Allard ..75/493,052,936 9/1962 Hamilton.. .164/56 3,158,466 11/1964 Muller ..75/583,201,224 8/1965 Grim ..75/49 X 3,251,680 5/1966 Goss et al ..75/5l X3,269,828 8/1966 Hale ..75/58 X 3,314,782 4/1967 Amaud ..75/58 X3,389,989 6/1968 Finkl ..75/49 3,501,290 3/1970 Finkl et al ..75/49 XPrimary Examiner-Henry W. Tarring, ll Attorney-Joseph J. OKeefe [5 7]ABSTRACT This invention is an improvement in a method for producingalloy steel which may contain, for example about 1.00% carbon, about0.45% manganese, about 0.25% silicon and 1.50% chromium. The steel has alower incidence of nonmetallic inclusions than steel produced in aconventional method, for example an air-melt electric furnace, openhearth. The steel is melted and tapped and then vacuum treated by anyone of several well-known methods. The steel is deoxidized and afluoride-containing acid slag is added to the steel. The steel and theslag are intimately mixed. The steel is then teemed into molds which canbe relatively large.

2 Claims, No Drawings METHOD FOR PRODUCING BEARING GRADE ALLOY STEELSBACKGROUND OF THE INVENTION Alloy steels intended for use in many highlystressed applications, for example, bearing races, are produced toexacting cleanliness specifications. Production of steels of this typeusually include the addition of a'solid deoxidant, such as aluminum,silicon, calcium-silicon, calcium-manganese-silicon, aluminum-siliconalloys or aluminum-silicon-manganese alloys, to the liquid steel todeoxidize the steels prior to teeming into molds. The solid deoxidantsreact with oxygen to form nonmetallic inclusions, for example, alumina,silica and com plex alumino-silicates which may become entrapped in thesteel as it solidifies. The nonmetallics so entrapped become foci forfatigue failure, thereby making these steels susceptible to earlyfailure by fatigue.

Several methods have been used to produce steels having a lowerincidence of nonmetallic inclusions therein. One such method includesmelting the steels in an initial air refining process and a remeltingand refining process in a vacuum arc furnace. Another method which hasbeen used is vacuum treatment of the steel after air refining. Both ofthese methods have been partially successful. However, the cleanlinessof steels produced by these processes has been erratic.

Steels of the bearing grade type were then teemed into relatively smallingot molds to prevent the formation of coarse carbides in the grainboundaries, to prevent carbon segregation in the top and center portionsof the ingot and to increase the rate of solidification of the steels tothereby decrease the size of nonmetallic inclusions formed in theingots. When teemed into larger molds the carbides and carbonsegregation cause weaknesses in the ingots and may cause the ingots tooverheat in the top and center portions of the ingot when heated for hotworking, making the ingot susceptible to bursting during hot working.

It is an object of this invention to provide an improved method forproducing substantially clean alloy steels in which the coarse carbidesand carbon segregation are reduced to a minimum of nonmetallicinclusions in the ingots.

It is also an object of this invention to provide an improved method forproducing alloy steels which will permit the steels to be teemed intorelatively large ingot molds.

SUMMARY OF THE INVENTION Broadly, the method of the invention includesmelting alloy steels in an air-melt furnace, for example, electricfurnace, by well-known methods, vacuum treating the steel, adding afluoride-containing acid slag to the steel after vacuum treatment,intimately mixing the steel and the slag and teeming the steel intomolds ofany size and type.

PREFERRED EMBODIMENT OF THE INVENTION l have found that alloy steel andin particular bearing grade alloy steel, having a decreased incidence ofnonmetallic inclusions therein may be produced by melting and refiningthe steel in a conventional air-melt metallurgical furnace, such as abasic electric furnace. The liquid steel is tapped into a conventionalladle. The liquid steel is then vacuum treated by any one of severalconventional methods. An acid slag is added to the liquid steel aftervacuum treatment. The liquid steel and the acid slag are intimatelymixed so that substantially all the liquid steel will come into contactwith the acid slag. At least one of several strong well-known soliddeoxidants is added to the steel to remove substantially all oftheoxygen remaining in the liquid steel. Steels produced by this method maybe teemed into relatively large ingot molds.

An alloy steel, for example a bearing grade steel containing about 0.90to about 1. percent carbon, about 0.30 to about 0.60 percent manganese,up to about 0.25 percent silicon, about 0.75 to about 2.00 percentchromium, the remainder incidental impurities, such as phosphorus andsulfur, may be refined in a conventional furnace by conventionalrefining methods. The liquid steel is tapped from the furnace into afurnace ladle in the usual manner. The liquid steel is exposed to avacuum of less than about 1 mm. of mercury. The method for vacuumtreating the'steel may be any one of several well known methods. Onesuch method is described in US. Pat. No. 3,019,496 entitled VacuumCasting Apparaus" issued Feb. 6, 1962 to H. C. Bigge. Of course, it ispossible to vacuum treat the liquid steel in the furnace ladle byplacing an appropriate apparatus atop the furnace ladle and applying avacuum thereto. Vacuum treatment decreases the oxygen level in theliquid steel. After melting, the oxygen in the liquid steel may be about0.003% to about 0. l5% Vacuum treatment will reduce the oxygen contentto between about 0.004% to about 0.0063%. Because some oxygen remains inthe liquid steel after treatment by the above described method, a soliddeoxidant, such as aluminum or silicon or alloys and mixtures thereof,may be added to the liquid steel to remove a substantial portion if notall of the oxygen remaining in the steel. The solid deoxidant reactswith oxygen in the steel to form nonmetallic inclusions such as alumina,silica and complex alumino-silicates.

In order to reduce the number of the aforementioned nonmetallicinclusions in the steel to a minimum, an acid slag containing silica anda fluorine-containing compound, for exam ple cryolite (Na AlF or sodiumfluoride (NaF) is added to the liquid steel in the ladle. The acid slagmay contain, for example 3 parts of silica to one part of thefluorine-containing compound on a weight basis. It is preferred to usecryolite in the acid slag: The steel and acid slag are intimately mixed.The mixing may be accomplished by bubbling an inert gas, for exampleargon, upwardly through the liquid steel after vacuum treatment. Theladle may be equipped with an electrical induction coil whereby theliquid steel is stirred by a flow of magnetic flux in the liquid steel.The acid slag absorbs a substantial portion, if not all of thenonmetallic inclusions in the liquid steel.

Mixing may also be accomplished by placing the acid slag on the bottomof a teeming ladle placed in the vacuum chamber prior to the vacuumtreatment. The liquid steel is poured from the furnace ladle into thevacuum chamber where it is received in the teeming ladle. The simpleexpedient of allowing the liquid steel to drop into the teeming ladleafter it has been exposed to the vacuum near the top of the vacuumchamber, causes the liquid steel to be intimately mixed with the slagwhich absorbs a substantial portion of the nonmetallic inclusions in theliquid steel.

Alloy grade bearing steel is usually teemed into 24 inch diameter molds.However, steel made according to the instant invention may be teemedinto ingot molds as large as 32 inches in diameter with no detrimentaleffects due to carbide coarsening or network formation in the grainboundaries, carbon segregation in the top and center portions of theingot and formation of unusually large nonmetallic inclusions formedbecause of decreased rate of solidification. Because of the fewer numberof molds required, mill yields are increased and the time required formold preparation and ingot stripping are kept at a minimum.

Whenever percentages are noted in this specification and claims, suchpercentages are on a weight basis unless otherwise noted.

In a specific example of the invention an alloy steel was prepared in aconventional manner in an electric furnace.

The steel had the following chemical compositions:

Percent 0 Mn P S Si Or A nace ladle. About 20 pounds of CaMnSi alloy,150 pounds of 5 Sio and 50 pounds of cryolite were then added to theliquid steel in the teeming ladle. The steel and slag were intimatelymixed for 6 minutes by bubbling about l0 cu. ft. per minute or argon gasupwardly through the liquid steel. The liquid steel was removed from thevacuum chamber and teemed into 32- inch :75 molds. The steel was hotworked into bars and billets suitable for shipment.

I claim:

1. In a method for reducing the amount of nonmetallic inclusions in analloy steel wherein, sequentially, said alloy steel is refined under abasic slag in an air-melt furnace, said alloy steel and said basic slagare tapped into a furnace ladle and said alloy steel is vacuum treatedby placing a teeming ladle into a vacuum chamber sealing said vacuumchamber by placing an airtight cover seal thereon, said airtight coverseal having an entry port into said vacuum chamber covered by a meltablematerial, placing said furnace ladle containing said alloy steel andbasic slag atop said airtight cover seal in a manner that will allowsaid alloy steel to flow downwardly into said teeming ladle in saidvacuum chamber when a vacuum is formed in said vacuum chamber, theimprovement comprising: placing an acid slag consisting of 3 parts ofsilica to l part of cryolite into said teeming ladle prior to sealingsaid vacuum chamber whereby when said vacuum is formed in said vacuumchamber, said alloy steel will flow downwardly from said furnace ladleinto said teeming ladle and contact said acid slag to cause thenonmetallic inclusions in said alloy steel to be adsorbed by said acidslag and terminating vacuum treatment of said alloy steel in said vacuumchamber prior to the passage of said basic slag in said furnace ladleinto said teeming ladle.

2. In a method for manufacturing an alloy steel in which said steel isrefined under a basic slag in an air-melt furnace, said steel is tappedinto a furnace ladle, said furnace ladle is placed in a vacuum chamberwhich is sealed by placing a sealing cover over said vacuum chamber,said sealing cover having a port covered by a removable cover throughwhich materials can be charged into said steel in said vacuum chamber,forming a vacuum in said vacuum chamber for a time to reduce the gaseouscontents of said steel while said steel is being agitated, theimprovement comprising: terminating the vacuum treatment and charging anacid slag consisting of 3 parts of silica to 1 part of cryolite saidsteel through said port in said sealing cover while said refined steelis being agitated to cause said steel and said acid slag to be inintimate contact with each other and nonmetalic inclusions in said steelto be adsorbed by said acid slag.

- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.39,117 Dated Februar 1, 1972 Albert L.- Lehman Inventor(s) It iscertified thst error appears in the above-identified patent and thatsaid Letters Patentar'e hereby corrected as shown below:

T Column 1, line l9, "lower't' should read --l0w--.

Column 2, line 12, "0.15%" should read "015%". Column 2, 1121913,"0.00%" should read -.OOl Column 3-, line, 6, "Siog" should read --SiOColumn 3, line 8, "or"should read --of-.

Column claim 2, line 21, after "cryol'ite" insert the word -into- Signedand. sealed this Ilth day of July 1972;

(SEAL) Attest:

EDWARD M.FLETGHER, JR. ROBERT GOTTSCHALK Attesting Officer 1Commissioner of Patents FORM PO-1050 (10-69) v USCOMM DC 6376 P69 m tru.s. eovsnumzm' PRINTING OFFICE 1989 0-366-334.

2. In a method for manufacturing an alloy steel in which said steel isrefined under a basic slag in an air-melt furnace, said steel is tappedinto a furnace ladle, said furnace ladle is placed in a vacuum chamberwhich is sealed by placing a sealing cover over said vacuum chamber,said sealing cover having a port covered by a removable cover throughwhich materials can be charged into said steel in said vAcuum chamber,forming a vacuum in said vacuum chamber for a time to reduce the gaseouscontents of said steel while said steel is being agitated, theimprovement comprising: terminating the vacuum treatment and charging anacid slag consisting of 3 parts of silica to 1 part of cryolite saidsteel through said port in said sealing cover while said refined steelis being agitated to cause said steel and said acid slag to be inintimate contact with each other and nonmetalic inclusions in said steelto be adsorbed by said acid slag.